1. Field of the Invention
The present invention relates to an Liquid Crystal Display (LCD) Device, and more particularly, to an in-plane switching mode LCD and a method of manufacturing the same.
2. Discussion of the Related Art
Twisted Nematic (TN) mode LCDs are generally used in current LCD devices. In the TN technique, electrodes are installed on each of two substrates and a liquid crystal (LC) director is arranged as a twisted 90°, then voltage is applied to the electrodes to drive the LC director.
However, TN mode LCDs have the disadvantage that the phase of light passing through the liquid crystal cell varies according to the direction of the light, causing a narrow viewing angle.
Recently, techniques have been actively developed for solving such a problem of the narrow viewing angle. Techniques for widening the viewing angle include a film-compensated mode for compensating the viewing angle with a compensating film, a multi-domain mode for dividing one pixel into several domains to vary the main viewing angle direction of each domain so that the pixel characteristic becomes the mean value of the several domain characteristics; an in-plane switching mode for applying a parallel electric field to twist the LC director in a plane parallel to an orientation film; a Vertical Alignment (VA) mode for using a negative liquid crystal and a vertically oriented film to vertically arrange the long axis of liquid crystal molecules about the oriented film; an Optically Compensated Birefringence (OCB) mode, and the like.
In the in-plane switching mode LCD, two electrodes are formed on one substrate for rotating the liquid crystal molecules in a plane parallel to the substrate. Voltage is applied between the two electrodes to induce an electric field parallel to the substrate, thereby reducing variation in birefringence of the liquid crystal.
Therefore, the in-plane switching mode LCD has excellent viewing angle characteristics compared to the TN mode LCD of the related art.
Hereinafter the in-plane switching mode LCD of the related art will be described with reference to the appended drawings as follows:
FIG. 1A is a plan view showing an in-plane switching mode LCD of the related art. FIG. 1B shows a cross section along line I-I′ in FIG. 1A, and FIG. 1C shows a cross section along line II-II′ in FIG. 1A.
FIG. 2A is a plan view for showing a general structure of an ITO-ITO electrode of the in-plane switching LCD. FIG. 2B shows transmitting and shielding areas of the ITO-ITO electrode of the in-plane switching LCD when positive DC voltage is applied to a data electrode. FIG. 2C shows the transmitting and shielding areas of the ITO-ITO electrode of the in-plane switching LCD when negative DC voltage is applied to the data electrode.
The general in-plane switching mode LCD as shown in FIG. 1A comprises data lines 10 and gate lines 11 arranged on a first substrate for defining a pixel area, a common line 12 arranged in the pixel area parallel to the gate lines 11, a thin film transistor placed in intersecting regions of the gate lines 11 and the data lines 10, data electrodes 14 arranged parallel to the data lines 10 in the pixels, common electrodes 13 extended from the common lines 12 and arranged between the data electrodes 14, and storage electrodes 55 extended from the data electrodes 14 and formed in the upper parts of the gate lines 11.
Referring also to FIG. 1B, the in-plane switching mode LCD is formed by joining the first substrate 18 and the second substrate 19 together in opposition to each other and injecting a liquid crystal layer 30 between the two substrates. The gate lines 11 are formed parallel to the common lines 12 on the first substrate 18. The common electrodes 13, which extend from the common lines 12, are commonly formed with the common lines 12. Here, a portion of the gate lines 11 functions as gate electrodes of the thin film transistor.
Then, a silicon nitride (SiNx) or a silicon oxide (SiOx) film is applied to the surface, including the gate lines 11 and the common electrodes 13, to form a gate insulation film 20, and a semiconductor layer 15 is formed on a portion of the gate insulation film as an active layer.
Next, the data lines 10 are formed on top of the gate insulation film 20 to form a matrix shape with the gate lines 11, and source/drain electrodes 16 and 17 are simultaneously formed to extend from the data lines 10 and be placed on a semiconductor layer 15. Here, the data electrodes 14 parallel to the common electrodes 13 and the storage electrodes 55 are formed at the same time, which connect the data electrodes 14.
The gate electrodes, the gate insulation film 20, the semiconductor layer 15, the source/drain electrodes 16 and 17 form the thin film transistor.
Then, a silicon oxide film, a silicon nitride film or an organic insulation film such as a BCB (Benzocyclobutene) film is applied on the surface, including the data lines 10, to form a protective film 25.
In the in-plane switching LCD, the common electrodes and the data electrodes can be formed on different planes, with the insulation film sandwiched as above or can be formed on one plane.
Also, the common electrodes and the data electrodes can be formed simultaneously with the lines made of metals including Cu, Al, Cr, Mo, Ti, Al alloy and the like for shielding a light, or a transparent conductive material such as ITO (Indium Tin Oxide) can be used in forming the same by further using a mask. When forming the electrodes using a mask, however, care should be used to avoid a short between the lines or electrodes.
When the data electrodes 14 and the common electrodes 13 are formed of ITO, which is a transparent conductive film that is excellent in transmitting light, the LCD is called an ITO-ITO in-plane switching LCD. A general structure of an ITO-ITO electrode is shown in FIG. 2.
A black matrix 21 is formed on the second substrate 19 to prevent light leakage, and an R, G, B color filter layer 22 is formed between the black matrix 21.
An overcoat layer 23 is formed on top of the color filter layer 22 to protect and planarize the color filter layer 22.
The ITO-ITO in-plane switching LCD formed as above has a horizontal or parallel electric field rather than a vertical electric field between the data and common electrodes, a vertical electric field in a middle portion of the electrodes, and horizontal or parallel and vertical electric fields commonly formed at corners of the electrodes.
Initially, the liquid crystal molecules between the electrodes are rotated parallel to the substrate due to a side electric field. After a certain time period, the liquid crystal molecules on the electrodes are rotated due to the vertical and side electric fields and an elastic force of the liquid crystal at the electrodes.
In an ITO-ITO in-plane switching LCD, the liquid crystal molecules on the electrodes have an orientation according to positive or negative voltage applied to the electrodes, thereby causing light transmissivity to be different at the data electrodes and the common electrodes.
In other words, when a positive DC voltage is applied to the data electrodes and a negative DC voltage is applied to the common electrodes, light is transmitted in a larger amount at the common electrodes than at the data electrodes, as shown in FIG. 2B. Also, when a negative DC voltage is applied to the data electrodes and a positive DC voltage is applied to the common electrodes, light is transmitted in a larger amount at the data electrodes 14, as shown in FIG. 2C.
In other words, a luminance difference is created at each electrode depending on whether a positive or negative voltage is applied.
As shown herein before, the foregoing in-plane switching LCD of the related art has the following problems.
To reduce degradation due to malfunction of the liquid crystal and to improve definition (picture quality), the common electrodes at the outermost position in the pixels are opposed and shielded with the black matrix layer on the substrate. Thus, the common electrodes and the data electrodes in a unit pixel have different the transmitting areas, which causes a luminance difference according to positively or negatively applied voltage. Flickers and residual images are generated because of the luminance difference, thus lowering reliability of the display device.